In the anatomical position the tendon of this powerful muscle curls round the inferior aspect of the neck of the femur to its insertion into the greater trochanter. A component of its action is therefore one of external rotation. However in flexion the tendon eventually comes to lie parallel with the neck of the femur; it is then purely an external rotator. During an action as when rising from a sitting posture, it is commonly thought that the head of the femur is stressed backwards so as to produce an internal rotation strain at the shaft neck junction and thereby, in a prosthetic replacement, a potentially prejudicial torque is created around the shaft of the femoral component. It is suggested that the action of the obturator externus, in particular, prevents this by converting the torque into compression along the neck of the natural femur or of a prosthesis; this is a condition which is suited to the material properties of bone in the natural state. This paper contrives to demonstrate the evidence for the mechanism described. Obturator externus is less frequently violated than other muscles around the hip; however, it is suggested that all of these should be carefully preserved or repaired effectively because they all contribute to a similar conversion of otherwise bending forces around the neck, to compression. In our quest to identify minutiae that contribute to long term survival of hip replacements, this may be another factor for consideration.
During revision hip surgery, damage to the sciatic nerve is due most commonly to excessive tension. While the nerve is strong in tension and is able to tolerate this remarkably, it is sensitive to lateral compression against angled structures and it is likely that tension causes injury by such pressure. In a personal series of 441 revision hip procedures, sciatic nerve injury occurred in 9 recognised cases: 2 were complete with no useful recovery 2 were severe with some incomplete recovery 3 were predominantly lateral popliteal with incomplete recovery 2 were transient and clinically fully recovered. In only two of these were the cause and the time of injury identifiable. In these cases, pain was not a serious feature but in eight separate medico-legal cases, burning pain of variable distribution has been the most serious complaint. Medical negligence has been successfully sustained on the grounds of We have found that in normal individuals the range of straight leg raising varies between 30 and 90 degrees. Towards the limit of this range the nerve is tight and a crude cadaveric test indicated a tension force of 40 newtons! In an athlete this may be extreme and yet the nerve does not complain. Excluding direct compression (e.g. by a retractor) and thermal injury, we suggest that the tension developed during unguarded straight leg raising while the patient is still anaesthetised is a serious risk factor. After even conservative lengthening, the tension rises alarmingly during such action. It is possible that pre-existing lumbar degenerative disk disease can contribute to the hazard and be source of pain.
An acute phase of periprosthetic bone loss occurs following total hip arthroplasty (THA). Periprosthetic bone loss undermines implant support, may contribute to its failure, and complicates revision surgery as allograft may be required to replace lost bone. We assessed the effect of a single 90mg dose of the bisphosphonate pamidronate on early periprosthetic bone mineral density (BMD), biochemical markers of bone turnover, and clinical outcome in 47 men and women undergoing hybrid THA in a randomised, double-blinded, placebo-controlled trial. The mean (± 95% CI) differences in BMD (area under BMD change.time curve) between those receiving pamidronate and those receiving placebo was 0.91(± 0.51) g.weeks/cm2 for the proximal femur (P=0.002), and 0.80 (±0.60) g.weeks/cm2 for the pelvis (P=0.009). Patients in the pamidronate group had suppression of all biochemical markers of bone turnover compared to placebo (P<
0.05), except for urinary free deoxypyridinoline. Both treatment groups experienced similar improvement in Harris hip and SF-36 UK outcome scores. The frequency of adverse events was similar in each treatment group (placebo 7/24, pamidronate 8/23, P>
0.05). Acute periprosthetic bone loss following THA is due to a transient increase in bone turnover. A single dose infusion of pamidronate in the early post-operative period significantly reduces this bone loss, and is well tolerated.
